Tapered light wave guide and wavelength converting element using the same

ABSTRACT

A tapered light wave guide reduced in propagation loss, improved in coupling efficiency and free from the problem of optical damage. An input section, a widthwise tapered coupling section having a depth d2, and a wave guide having a depth d1 are formed on an LiNbO 3  substrate. A depthwise tapered section in which the depth is changed from d2 to d1 is provided to connect the widthwise tapered coupling section having constant depth d2 and the wave guide, whereby a reduction in light propagation efficiency due to optical damage is prevented.

BACKGROUND OF THE INVENTION

This invention relates to a light wave guide which has a taperedcoupling section and which enables highly efficient coupling between alight wave guide and a laser or a fiber used in the field of opticalinformation processing and in the field of applied optical measurementcontrol applied with a coherent light source, and to a wavelengthconverting element using this wave guide.

TE/TM mode splitters and wavelength converting elements haveconventionally been manufactured by forming a light wave guide on LiNbO₃based on a proton exchange method. In wave guides thereby manufactured,the difference between the refractive indices of the wave guide and thebase is large (Δne>0.1), and the thickness is very small, 0.4 to 0.6 μmif the wave guide is arranged to propagate light in a single mode. Toimprove the efficiency of coupling with the wave guide, therefore,tapering a section through which light is introduced into the wave guidehas been proposed.

A conventional light wave guide is known in which a section throughwhich incident light is introduced into a wave guide section is widen bytapering or the like (This type of light wave guide is hereinafterreferred to simply as "tapered wave guide"). FIG. 10 shows the basicconstruction of this conventional tapered wave guide. This wave guideincludes a ferroelectric material substrate 21, a wave guide 22, atapered wave guide 23, and an input section 24.

In the conventional tapered wave guide having this construction, theshape and the area of the section through which light is introduced arechanged so that the distribution of the wave front of exciting coherentlight in the wave guide matches with the electric field of the guidedmode of propagation through the wave guide, thereby obtaining a highefficiency of coupling with the wave guide with respect to coherentlight.

A method of manufacturing such a tapered wave guide is disclosed by J.C. Campbell on pages 900 to 902 of Applied Optics, March 1979, volume18. In this method, as shown in FIG. 11, a tapered incident light waveguide is formed by gradually immersing substrate 21 in a solution 25 ofAgNO₃ so that the diffusion depth is changed. In the case of forming awave guide on LiNbO₃ substrate 21, benzoic acid is used as solution 25and heating is effected at about 200° C. to form a tapered wave guide.In FIG. 11, a heater and a beaker are indicated by 26 and 27,respectively.

In the above-described arrangement, however, it is difficult tosufficiently widen the input section 24 of the tapered incident lightwave guide relative to the non-tapered wave guide. While the wave guideis widened in directions of width and depth, the section through whichlight propagates is abruptly reduced at a position where the wave guideis narrowed by tapering. Therefore the power density in the wave guideis abruptly increased at this position so that optical damage is caused,if the light is converged at this position. In the tapered section, boththe width and the depth are increased so that a multi-mode wave guide isformed, but coupling occurs between guided multiple modes by a change inrefractive index caused by optical damage, resulting in occurrence of acoupling loss at the position where the single-mode wave guide and thetapered section are connected.

The above-described method of manufacturing a tapered wave guide entailsa problem in that the temperature of a portion not immersed in thesolution is lowered by vapor during the high-temperature heat treatment,and in that the section through which light is introduced cannot beformed in accordance with the design with desired reproducibility.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a tapered light waveguide having reduced in propagation losses, having a high couplingefficiency and free from the problem of optical damage.

It is another object of the present invention to provide a wavelengthconverting element formed by using this wave guide and having highconversion efficiency.

To achieve these objects, according to one aspect of the presentinvention, there is provided a tapered light wave guide comprising awave guide formed on a substrate and having a depth d1; a couplingsection formed at an end of the wave guide, tapered in the direction ofwidth of the substrate and having a depth d2 (d2>d1); and a depthtapered section formed between the wave guide and the coupling section,the depth of the depthwise tapered section being changed from d2 to d1by tapering.

According to another aspect of the invention, there is provided awavelength converting element comprising an ion-exchanged wave guideformed on a nonlinear optical crystal substrate; and having a depth d1;a coupling section formed at an end of the ionexchanged wave guide,tapered in the direction of width of the substrate and having a depth d2(d2>d1); and a depthwise tapered section formed between the wave guideand the coupling section, the depth of the depthwise tapered sectionbeing changed from d2 to d1 by tapering.

Preferably, if the width of the coupling section is W2, the relationshipbetween the depth d and the width W2 of the coupling section withrespect to the guided light power P is P/(d2×W2)≦300 kW/cm².

In the arrangement of the present invention, a depthwise tapered sectionin which the depth is changed from d2 to d1 is provided between the waveguide formed on the substrate and having the depth d1 and the couplingsection formed at the end of the wave guide, tapered in the direction ofwidth of the substrate and having the depth d2 (d2>d1). To avoidoccurrence of optical damage at the coupling section, the wave guidedepth is increased in the widthwise tapered section of the light waveguide so as to reduce the power density therein, and the light waveguide is tapered so that the depth is reduced from the position wherethe widthwise tapered section ends and where the propagation mode isshifted to a single mode. It is thereby possible to construct a taperedwave guide having reduced propagation losses, improved in couplingefficiency and free from the problem of optical damage. A wavelengthconverting element having improved conversion efficiency can bemanufactured with this light wave guide having improved couplingefficiency.

These and other objects and advantages of the present invention will beapparent from the following detailed description, taken in conjunctionwith the accompanying drawings of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view illustrating a tapered wave guidein accordance with a first embodiment of the present invention;

FIG. 2A is a diagram showing guided modes and changes in refractiveindex of the conventional tapered wave guide;

FIG. 2B is a diagram showing guided modes and changes in refractiveindex of the tapered wave guide of the present invention;

FIGS. 3A to 3D are sectional perspective views for explaining steps ofmanufacturing the tapered wave guide in accordance with the firstembodiment of the present invention;

FIG. 4 is a diagram showing the results of measurement of couplinglosses in the tapered wave guide in accordance with the first embodimentof the present invention;

FIG. 5 is a schematic perspective view illustrating a wavelengthconverting element in accordance with a second embodiment of the presentinvention;

FIG. 6 is a perspective view illustrating a wavelength convertingelement in accordance with a third embodiment of the present invention;

FIG. 7 is a diagram showing changes in a second harmonic generationoutput from the wavelength converting element of the rpesent invention;

FIGS. 8A and 8B are diagrams showing states of light guided in thewavelength converting element of the present invention;

FIG. 9 is a diagram showing the relationship between the wave guidedepth and damage withstanding capacity of the wavelength convertingelement of the present invention;

FIG. 10 is a schematic perspective view of the conventional tapered waveguide; and

FIG, 11 is a schematic diagram showing a method of manufacturing theconventional tapered wave guide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

FIG. 1 shows the construction of a tapered wave guide in accordance witha first embodiment of the present invention. A substrate 1 is formed ofLiNbO₃ to provide a +Z plate (the + side of the substrate cutperpendicularly to the Z axis) having a refractive index of 2.1. On thesubstrate 1 are provided a widthwise tapered section 2 formed by protonexchange in pyrophosphoric acid, having a refractive index of 2.3 and adepth of 1.5 μm and tapered widthwise, a wave guide 3 formed by protonexchange in pyrophosphoric acid and having a depth of 0.4 μm, adepthwise tapered section 4 formed by proton exchange in pyrophosphoricacid and tapered so that its depth is changed from 1.5 to 0.4 μm, and aninput section 5 formed at an end surface by optical polishing.

A constructional feature of this embodiment, in which the input section5 having a depth d2, the widthwise tapered section 2 having a depth d2and the wave guide 3 having a depth d1 are arranged on the LiNbO₃substrate 1, resides in that the depthwise tapered section 4 in whichthe depth is changed from d2 to d1 is provided to connect the widthwisetapered coupling section 2 having the constant depth d2 and the waveguide 3 having the depth d1.

The importance of setting the depth of the widthwise tapered couplingsection to a constant depth of d2 and providing depth tapered section 4in the wave guide 3 will be explained below with reference to thedrawings.

FIG. 2A is a perspective view of a conventional wave guide having atapered section 2a changed in both depthwise and widthwise directions,and FIG. 2B is a perspective view of the wave guide in accordance withthis embodiment in which the tapered portion is separated into a section2 tapered in the widthwise direction and a section 4 tapered in thedepthwise direction.

Referring to FIG. 2A, a loss is caused at the tapered section 2a by achange in refractive index due to optical damage. That is, the waveguide width of the tapered section 2a is increased relative to a waveguide 7a so that this section is formed as a multimode wave guide, andso that the mode of light guided is shifted from a fundamental mode to amultimode by a change in refractive index caused by optical damage.Since the wave guide section 7a has a wave guide width such as toconstitute a single-mode wave guide which propagates light in thefundamental mode alone, a mode mismatch occurs between the multimodeoccurring at the tapered section 2a and the single-mode wave guide,thereby causing a loss. This phenomenon will be described below withrespect to wave guide modes.

In (a) of FIG. 2A, a fundamental mode of light excited at an inputsection 5a is shown. In the tapered section 2a, both width and depth ofthe wave guide are reduced, so that the light power density isincreased. If optical damage is thereby caused, the refractive indexdistribution changes as indicated in (b) and the wave guide mode isshifted from the fundamental mode to a multimode.

Thereafter, in the state shown in (c) of FIG. 2A, the extent of shiftingto the multimode is increased. However, the wave guide section 7aconstitutes a singlemode wave guide which propagates light in afundamental mode alone as shown in (d), and a mismatch therefore occursbetween the conditions (c) and (d), thereby causing a large loss ofcoupling between these wave guide sections.

To prevent such a coupling loss, it is necessary to limit theconcentration of power density at the tapered section 3a and to preventto optical damage. According to the present invention, a widthwise taperand a depthwise taper are provided separately, and the depthwise taperis set in the wave guide, so that the cross-sectional area of thewidthwise tapered section is increased and the light power density inthe widthwise tapered section is reduced. It is thereby possible toprevent optical damage. This effect will be described below withreference to FIG. 2B.

With respect to the wave guide mode started from the fundamental mode asshown in (e) of FIG. 2B, the increase in light power density is limitedand no optical damage is caused, because the widthwise tapered section 2is narrowed in the widthwise direction alone and has a larger sectionalarea in comparison with the conventional arrangement. The lighttherefore propagates in the fundamental mode, as shown in (f) and (g).

At the end of the widthwise taper, no multimode occurs even if opticaldamage is caused, since the wave guide has a single mode in thewidthwise direction. It is possible to connect the widthwise taperedsection to the wave guide 7 through the depthwise tapered section 4 toform a tapered wave guide having a small loss.

A method of manufacturing the thus-constructed tapered incident lightwave guide in accordance with the first embodiment will be describedbelow. A tapered wave guide in accordance with the present invention wasmanufactured by utilizing a method disclosed by the inventors of thepresent invention in Japanese Patent Unexamined Publication No.2-236505. The manufacture process will be described below with referenceto FIGS. 3A to 3D.

In the step shown in FIG. 3A, a protective Ta₂ O₅ mask 32 for forming acoupling section was formed on a+Z plate LiNbO₃ substrate 31 by electronbeam deposition to have a thickness of 300 ↑. Next, a photoresistpattern having a thickness of 0.5 μm was formed on the protective mask32 by ordinary photolithography, the protective Ta₂ O₅ mask 32 wasthereafter etched by CF4, and then the photoresist was removed. Next, anLiNbO₃ mask substrate 33 having a thickness of 1 mm and having two endsurfaces processed by optical polishing was provided as a secondprotective mask and was pressed on the LINbO₃ mask and fixed by a jig.

In the step shown in FIG. 3B, proton exchange was effected inpyrophosphoric acid at 230° C. for 270 minutes to form a couplingsection 35 having a refractive index of 2.3 and a depth of 1.5 μm. Inthis step, while the wave guide depth at the coupling section 35 is setto a certain value (1.5 μm in this case) determined by the time forproton exchange in pyrophosphoric acid, tapering in the direction ofdepth is also effected at a position immediately below the masksubstrate 33, since pyrophosphoric acid also penetrates into a portioncovered with the mask substrate 33.

In the step shown in FIG. 3C, after the mask substrate 33 had beenremoved, the substrate 31 underwent proton exchange in pyrophosphoricacid at 230° C. for 5 minutes to form a high-refractivity wave guide 36having a refractive index of 2.3 and a depth of 0.4 μm. In this step,since a wave guide having a certain depth (equal to or smaller than 1.5μm) is formed at the connection between the coupling section 35 and thewave guide 36 in the step shown in FIG. 3B, the wave guide 36 issmoothly tapered in the depthwise direction when formed by protonexchange.

In the step shown in FIG. 3D, the Ta mask 32 was removed and the surfaceperpendicular to the wave guide is finished by optical polishing.

This method enables the wave guide to be easily designed by adopting theprocess of forming with a mask the coupling section tapered widthwise ofthe wave guide. Also, the manufacturing tolerance and the yield areincreased because widthwise tapering and depthwise tapering areperformed separately. That is, the manufacturing tolerance was increasedand the manufacturing yield was increased by 10 to 50% in comparisonwith the conventional manufacturing method based on partial protonexchange.

The propagation loss of the wave guide manufactured was measured byexciting semiconductor laser light having a wavelength of 0.8 μm in thetapered incident light wave guide 2. To effect this measurement, lightfrom a semiconductor laser was condensed by a focusing optical systemincluding a collimator lens having a numerical aperture (NA) of 0.3, a1/2 plate and a condenser lens having a numerical aperture of 0.6 sothat a minimum focusing spot diameter of 5×1 μm was converged on thetapered wave guide. A fluorescent material was applied on the waveguide, and scattered light from the surface of the wave guide wasobserved with a streak camera to measure the propagation mode of thewave guide and the propagation loss of the same from the intensity ofscattered light from the wave guide.

As a result, no coupling loss due to optical damage was observed withrespect to the tapered incident light wave guide manufactured by themethod of the invention even when the power of guided light was 30 mW,while in the conventional tapered incident light wave guide a couplingloss occurred at the tapered coupling section due to optical damage whenthe power of guided light was 10 mW or greater.

FIG. 4 shows the results of the measurement of the coupling loss of thetapered incident light wave guide, the abscissa corresponding to adirection of a wave guide propagation component, the ordinaterepresenting the loss. The line (a) in FIG. 4 relates to the taperedwave guide, and the line (b) relates to the linear wave guide.

With respect to the linear wave guide, there are a loss of 2.3 dB ofcoupling with the light source and a guided loss of 0.9 dB, and thetotal loss is 3.2 dB. With respect to the tapered wave guide, there area coupling loss of 1.0 dB, a loss of 0.3 dB at the tapered section and aguided loss of 0.9 dB, and the total loss is 2.2 dB. Thus, by thetapered wave guide, the loss of the wave guide was reduced from 3.2 to2.2 dB by 1 dB.

Thus, in accordance with this embodiment, a tapered wave guide can beconstructed which can be easily designed and manufactured with respectto the coupling, which has improved in coupling efficiency and which isfree from the problem of optical damage.

In this embodiment, LiNbO₃ is used as the substrate material. However,the substrate may be formed of any other material, e.g., a ferroelectricmaterial such as LiNbO₃ doped with MgO, LiTaO₃, KNbO₃ or KTP, adielectric such as SiO₂, an organic material such as MNA, or a chemicalcompound semiconductor such as ZnS, as long as a wave guide can beformed on the substrate.

Embodiment 2

A second embodiment of the present invention will be described belowwith reference to FIG. 5. This embodiment is a light wavelengthconverting element in which the wave guide described as the firstembodiment is formed in a non-linear optical crystal.

In the construction of the wavelength converting element of the secondembodiment, the components identical to those described with respect tothe first embodiment are indicated by the same reference numerals. Thewavelength converting element shown in FIG. 5 has a substrate 1 formedof LiNbO₃ as a +Z plate (the + side of the substrate cut perpendicularlyto the Z axis) having a refractive index of 2.1, a widthwise taperedsection 2 formed by proton exchange in phosphoric acid, having arefractive index of 2.3 and a depth of 1.5 μm and tapered widthwise, awave guide 7 formed by proton exchange in pyrophosphoric acid and havinga depth of 0.4 μm, a depthwise tapered section 4 formed by protonexchange in pyrophosphoric acid and tapered so that its depth is changedfrom 1.5 to 0.4 μm, an input section 5 formed at an end surface byoptical polishing, a semiconductor laser 9, and a focusing opticalsystem 10 for introducing light from the semiconductor laser 9 to theinput section 5.

The operation of the thus-constructed wavelength converting element ofthe second embodiment will be described below.

A wave length converting element having an input section 5, a widthwisetapered section 2, a depthwise tapered section 4 and a wave guide 7formed by proton exchange on the non-linear optical crystal LiNbO₃substrate 1 was formed by the method described with respect to the firstembodiment. Light having a wavelength of 0.8 μm and a luminance area of5×1 μm was introduced from the semiconductor laser 9 into the inputsection 5. The efficiency of coupling between the semiconductor laser 9and the wave guide 7 measured was 50%. This value is 1.6 times greaterthan a coupling efficiency of 30% through the tapered wave guide and thefocusing optical system of the semiconductor laser of the conventionalarrangement. Also, based on the non-linear optical effect, a secondharmonic wave P2 having a wavelength of 0.4 μm was generatedorthogonally to the proton-exchanged wave guide by Cherenkov radiation,and a secondary harmonic component output P₂ of 0.2 mW could be obtainedwith respect to a semiconductor laser output of 40 mW. This value istwice as large as that of the conventional arrangement (secondaryharmonic component output of 0.1 mW with respect to semiconductor laseroutput of 40 mW). Further, an LiNbO₃ substrate of 6×2×2 mm, asemiconductor laser having a length of 200 μm and a focusing opticalsystem of 10×5×5 mm were integrally combined into a module, therebyforming a wavelength converting element having a small size, 20×6×6 mm.Thus, in accordance with this embodiment, a small large-outoutwavelength converting element can be formed.

In this embodiment, LiNbO₃ is used as a non-linear optical material ofthe substrate. However, the substrate may be formed of any othermaterial, e.g., a ferroelectric material such as LiNbO₃ doped with MgO,LiTaO₃, KNbO₃ or KTP, a dielectric such as SiO₂, an organic materialsuch as MNA, or a chemical compound semiconductor such as ZnS, as longas the substrate has a large non-linear optical constant.

Embodiment 3

FIG. 6 shows the construction of a wave length converting element inaccordance with a third embodiment of the present invention. Therelationship between the depth and the width of the coupling section andthe power of incident light, set to d2, W2, and P1, respectively, wasexamined.

A wave length converting element constructed in accordance with thethird embodiment as shown in FIG. 6 was manufactured, and the change inthe second harmonic generation (SHG) output P2 with respect to timebased on the non-linear optical effect was observed by introducing lightfrom semiconductor laser 9 into the manufactured wavelength convertingelement. FIG. 7 is a graph showing the results of measurements ofincident light power P1 and SHG output P2 with respect to time. As canbe read from this graph, no change is observed in the second harmonicSHG output P2 with time when the power of the guided light is 17 mW orless, but the SHG output P2 is changed after 5 to 10 minutes when theoutput is 20 mW or more.

It is thought that this change is caused by a coupling loss at thetapered coupling section between the input section and the wave guidesection due to optical damage to guided light in the proton-exchangedwave guide formed on the LiNbO₃ substrate. FIGS. 8A and 8B show theresults of observation of the states of guided light with a CCD camera;FIGS. 8A and 8B show the light receiving surface of the CCD camera after0 minute and after 10 minutes, respectively. The hatching in FIGS. 8Aand 8B indicates guided light, and indicate that the loss at the taperedsection is increased after 10 minutes so that the guided light does nottravel beyond the tapered section. Thus, it was confirmed that the SHGoutput fluctuated by the occurrence of a coupling loss due to opticaldamage.

Next, the relationship between the shape of the tapered section andoptical damage was obtained.

As shown in Table 1, the wave guide width W2 of the tapered section wasset to 4 μm, the width W1 and the depth d1 of the wave guide sectionwere set to 2 and 0.4 μm, respectively, while the depth d2 of thecoupling was changed by being set to 0.4, 1, and 1.5 μm. The power ofguided light in which optical damage was caused (which power ishereinafter referred to as Pth) was measured with respect to the depthof the coupling section. FIG. 9 shows the relationship between the waveguide depth and Pth along with the maximum of the guide light power inthe wave guide. As can be read from FIG. 9, the maximum of the guidedlight power in the wave guide is inversely proportional to Pth and, asthe wave guide size (W2×d2) is increased, pth increases so that themaximum of the guided light power is reduced. The value of Pth/(d2×W2)obtained from FIG. 9 is 300 kW/cm² irrespective of the depth of the waveguide. From this result, it is confirmed that if the size (W2×d2) of thecoupling section is selected so that Pth/(W2×d2)≦300 kW/cm², it ispossible to form a wave length converting element having reduced outputvariations due to optical damage, reduced losses, improved stability anda large output by virtue of high-efficiency coupling with thesemiconductor laser. Thus, in accordance with this embodiment, the shapeof the coupling section is limited to form a stable large-outputwavelength converting element.

According to the present invention, as described above, a taperedincident light wave guide can be constructed which has reducedpropagation losses, improved coupling efficiency and which is free fromthe problem of optical damage. This wave guide can be used veryadvantageously in practice.

In the wavelength converting element in accordance with the presentinvention, light from a semiconductor laser and a single-mode wave guidecan be coupled at a high efficiency. It is thereby possible to increasethe power density of light propagated through the wave guide and, hence,to greatly improve the wavelength conversion efficiency based on thenon-linear optical effect.

Moreover, it is possible to limit the increase in coupling loss due tooptical damage caused in the coupling section by designing thewavelength converting element so that the relationship between the depthd2 and the width W2 of the coupling section with respect the guidedlight power P is P/(d2×W2)≦300 kW/cm². Consequently, a stablelarge-output wavelength converting element can be formed and can be usedvery advantageously in practice.

                  TABLE 1                                                         ______________________________________                                                                              Pth                                     W.sub.2                                                                              W.sub.1   d.sub.1                                                                              d.sub.2 Pth   d.sub.2 *W.sub.2                        (μm)                                                                              (μm)   (μm)                                                                              (μm) (mW)  (kw/cm.sup.2)                           ______________________________________                                        4      2         0.4    0.4     4.8   300                                     4      2         0.4    1       12    300                                     4      2         0.4    1.5     18    300                                     ______________________________________                                    

What is claimed is:
 1. A tapered light wave guide comprising:a substratehaving a first end face and a second end face opposite to one anotherand a first side surface and a second side surface opposite to oneanother and intersecting said first end face and said second end face; awave guide formed on one of said first side surface and said second sidesurface, extending in a propagation direction of light through said waveguide and having a uniform width W1 and a uniform depth d1 in saidpropagation direction; a coupling section formed on said one of saidfirst side surface and said second side surface, extending from one ofsaid first end face and said second end face in said propagationdirection, being tapered widthwise of the substrate from a width W2(W2>W1) to said width W1 and having a uniform depth d2 (d2>d1); and adepthwise tapered section formed on said one of said first side surfaceand said second side surface to connect said coupling section and saidwave guide with each other, said depthwise tapered section having adepth which gradually changes from d2 to d1 by tapering.
 2. A taperedlight wave guide according to claim 1, wherein LiNbO₃ or LiTaO₃ is usedas a material of said substrate.
 3. A tapered light wave guide accordingto claim 1, wherein incident light is propagated in a fundamental modefrom said coupling section into said wave guide.
 4. A wavelengthconverting element comprising:a non-linear optical crystal substrate; aproton-exchanged wave guide formed on said substrate and having a depthd1; a coupling section formed at an end of said proton-exchanged waveguide, tapered widthwise of said substrate and having a depth d2(d2>d1); and a depthwise tapered section formed in said wave guide atthe position where said coupling section and said wave guide areconnected, the depth of said depthwise tapered section being changedfrom d2 to d1 by tapering.
 5. A wavelength converting element accordingto claim 4, wherein if the width of said coupling section is W2, therelationship between the depth d and the width W2 of said couplingsection with respect the guided light power P is P/(d2×W2)≦300 kW/cm².6. A wavelength converting element according to claim 4, wherein anLiNbO₃ or LiTaO₃ substrate is used as said non-linear optical crystalsubstrate.
 7. A wavelength converting element according to claim 4,wherein incident light is propagated in a fundamental mode from saidcoupling section into said wave guide, and second harmonic generationlight is emitted by Cherenkov radiation.